1 PROJECT SUMMARY/ABSTRACT 2 1 Physical activity is essential for healthy development in youth and prevention of chronic diseases such as 2 obesity, diabetes, cardiovascular disease, and some types of cancer. Youth typically need to accrue physical 3 activity across multiple settings, including at home, at school, in recreation areas, and through active travel, to 4 meet the 60 minutes per day guideline for total physical activity. Active travel to school, in particular, is 5 promising for improving youth health because (1) youth who regularly walk to school have an average of 15-20 6 more minutes per day of total physical activity, and (2) less than 25% of youth walk to school, suggesting 7 substantial room for improvement. Walking School Bus programs, which involve adult chaperones walking to 8 school with groups of students, are gaining popularity across the US and world, but little evidence exists on 9 implementation of Walking School Bus programs. Initial evidence suggests that levels of fidelity and 10 sustainability are highly variable in existing programs. The proposed study will use the Consolidated 11 Framework for Implementation Research (CFIR) and mixed-methods to systematically evaluate 12 implementation contexts (i.e., barriers, facilitators, and processes) and implementation outcomes (i.e., 13 implementation success) within existing Walking School Bus programs. Our team includes experts in youth 14 physical activity, implementation science, qualitative methods, and Walking School Bus practice. Our specific 15 aims are to (1) describe implementation contextual factors that distinguish between existing Walking School 16 Bus programs with high- vs. low- implementation success, and (2) determine whether key program descriptive 17 factors are associated with implementation success. A concurrent mixed-methods design will be used to merge 18 quantitative and qualitative data to maximize the ability to capture both breadth and depth of information. First, 19 we will conduct a brief survey of 200 existing Walking School Bus programs from across the US and world to 20 learn from a variety of approaches. Then, we will interview 12-15 programs with high-implementation success 21 and 12-15 programs with low-implementation success to capture in-depth information on factors related to 22 success. Interviews will be transcribed and content analyzed by two researchers to rate the impact of each 23 CFIR construct on implementation, and garner in-depth descriptions of the implementation factors. The 24 identified best practices, as related to implementation fidelity, penetration, and sustainability, will inform the 25 development of a set of implementation principles and quantitative measures to support a future R01 hybrid 26 implementation/effectiveness evaluation study of Walking School Bus programs across the US. Evidence- 27 informed guidance for implementing effective Walking School Bus programs could have meaningful impacts on 28 improving overall physical activity and population health in youth. Such programs could be better incorporated 29 into existing widespread Safe Routes to School efforts, which receive national funding, to provide a multi-level 30 approach (i.e., social and environmental) for supporting walking to school.